Abstract

Unenlagiinae is mostly recognized as a subclade of dromaeosaurids. They have the modified pedal digit II that characterize all dromeosaurids, which is typically related to predation. However, derived Laurasian dromaeosaurids (eudromaeosaurs) differ from unenlagiines in having a shorter metatarsus and pedal phalanx II-1, and more ginglymoid articular surfaces in metatarsals and pedal phalanges. Further, unenlagiines have a subarctometatarsal condition, which could have increased the mechanical efficiency during locomotion. All these discrepancies possibly reflect different locomotor and predatory habits. To evaluate this we conducted morphometric analyses and comparisons of qualitative morphological aspects. The former consisted in two phylogenetic principal component analyses, one of them based on lengths of femur, tibia and metatarsus, and width of metatarsus, and the other based on lengths of pedal phalanges. The data sampling covered several coelurosaurian and non-coelurosaurian taxa. The first analysis showed the unenlagiines close to taxa with long tibiae and long and slender metatarsi, which are features considered to provide high cursorial capacities. Instead, eudromaeosaurs are close to taxa with shorter tibiae and shorter and wider metatarsi, which can be considered with low cursorial capacities. The second analysis showed that eudromaeosaurs and unenlagiines have similar phalangeal proportions. Moreover, they share the elongation of distal phalanges, which is a feature related to the capacity of grasping. The shorter and wider metatarsus, more ginglymoid articular surfaces and a shorter pedal phalanx II-2 of eudromaeosaurs possibly allowed them to exert a greater gripping strength. Thus, they had the potential of hunting large prey. Instead, the longer and slender subarctometatarsus, lesser ginglymoid articular surfaces and a longer pedal phalanx II-2 of unenlagiines possibly gave to them greater cursorial capacities and the ability to hunt smaller and elusive prey on the ground. Thus, the different morphological evolutionary paths of dromaeosaurids lineages seem to indicate different locomotor and predatory specializations.

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